Li Battery Cathode Materials:
The positive electrode (cathode) material is the limiting factor in the production of high capacity and high energy density lithium ion batteries. An important class of cathode materials for secondary lithium batteries is constituted by rock-salt type layered lithium metal oxides of the general composition LiMO2, where M is a metallic species or a mixture of several such. In such layered oxides, every second plane in <111> direction (from F-3m cubic system) contains alternating lithium cations or cations of species M (M. S. Whittingham, Science 192 (1976) 1126-1127; M. S. Whittingham, Chemical Reviews 104 (2004) 4271-4302). It has been typical in the field of batteries to look for well-ordered layered cathodes, in which the Li and M cations are well separated in distinct (111) layers. For example, capacity degrading in LiNiO2 can be attributed to the migration of nickel cations to the lithium layer (C. Delmas et al., Journal of Power Sources 68 (1997) 120-125). Introducing Mn to the compound improves its layeredness and results in significantly better capacity retention (K. Kang et al., Science 311 (2006) 977-980). Similarly, cation mixing is believed to have a strong negative impact on the electrochemical performance of Li(Li,Ni,Mn,Co)O2 (X. Zhang et al., J. Power Sources 195 (2010) 1292-1301). The capacity of most well-ordered layered cathode materials has been limited to 150-180 mAh/g which corresponds to ≈0.5 to 0.65 Li ions per LiMO2 formula unit (T. Ohzuku, Y. Makimura, Chemistry Letters 30 (2001) 744-745; J. Choi, A. Manthiram, J. Electrochem. Soc. 152 (2005) A1714-A1718). To achieve higher capacity, complex overcharging schemes have been developed, but these are difficult to implement in the manufacturing of batteries. For example, some Li(Li,Ni,Co,Mn)O2 compounds are overcharged in the first cycle at a voltage above 4.7 V in order to release oxygen, and achieve a higher capacity in the subsequent cycles (M. M. Thackeray et al., J. Mater. Chem. 17 (2007) 3112-3125; A. R. Armstrong et al., J. Am. Chem. Soc. 128 (2006) 8694-8698). But this overcharge process is more expensive to implement and leads to cathode materials with limited long-term stability as well as reduced charge/discharge rate capability. Thus, overcharging may lead to oxygen evolution and poses a potential safety risk, and adds cost and complications to battery manufacturing.